Phase behavior of the microemulsions

Lv FF, Zheng LQ, and Tung CH. (2005). Phase behavior of the microemulsions and the stability of the chloramphenicol in the microemulsion-based ocular drug delivery system. International Journal of Pharmaceuticals, 301, 237-246. 

TX-lOO, and [bmim][PFJ. It was found that the phase behavior of the microemulsion could be easily identified by its dielectric response. The dielectric behavior of the IL microemulsion in the GHz range is consistent with that of TX-lOO/water mixtures with comparable water-to-TX-100 weight ratio. 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

Recently, the phase equilibria of a microemulsion were reported. The phase behavior of a microemulsion formed with food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) was studied. Critical microemulsion concentration (cpc) was deduced from the dependence of the pressure of cloud points on the concentration of... 

For a given surfactant, the ability to form a single-phase w/o microemulsion is a function of the type of oil, nature of the electrolyte, solution composition, and temperature (54-58). When microemulsions are used as reaction media, the added reactants and the reaction products can also influence the phase stability. Figure 2.2.4 illustrates the effects of temperature and ammonia concentration on the phase behavior of the NP-5/cyclohexane/water system (27). In the absence of ammonia, the central region bounded by the two curves represents the single-phase microemulsion region. Above the upper curve (the solubilization limit), a water-in-oil microemulsion coexists with an aqueous phase, while below the lower curve (the solubility limit), an oil-in-water water microemulsion coexists with an oil phase. It can be seen that introducing ammonia into the system results in a shift of the solubilization... 

In this article we describe the phase behavior of a microemulsion system chosen for the free radical polymerization of acrylamide within near-critical and supercritical alkane continuous phases. The effects of pressure, temperature, and composition on the phase behavior all influence the choice of operating parameters for the polymerization. These results not only provide a basis for subsequent polymerization studies, but also provide data on the properties of reverse micelles formed in supercritical fluids from nonionic surfactants. 

Investigation of the phase behavior of the Brij-based microemulsion system in ethane/propane mixtures defines the operating conditions for the polymerization process and provides evidence of formation of stable microemulsions in supercritical... 

Our quantitative findings concern the phase behavior of the L and microemulsion systems are summarized in Figure 3. In the absence of alcohol, the maximum MMA content of the Lj phase is fixed by a limiting MMA/SLS mole ratio of approximately three. Higher mole ratios will invariably lead to a two-phase system. With alcohol... 

This chapter covers the fundamentals of surfactant flooding, which include microemulsion properties, phase behavior, interfacial tension, capillary desaturation, surfactant adsorption and retention, and relative permeabilities in surfactant flooding. It provides the basic theories for surfactant flooding and presents new concepts and views about capillary number (trapping number), relative permeabilities, two-phase approximation of the microemulsion phase behavior, and interfacial tension. This chapter also presents an experimental study of surfactant flooding in a low-permeability reservoir. 

The preceding discussions focus on the phase behavior of the middle-phase microemulsion. Actually, when an alkali is added to a surfactant system, a mixed phase is formed. It takes some time for a clear middle-phase microemulsion in equilibrium to be formed. The process could take a long time, from several days to weeks. Sometimes, a cloudy middle phase (without a middle-phase microemulsion) is formed between the upper oil phase and lower water phase. The mixed phase has several characteristics (Li et al., 2002) ... 

Penders, M.H.G.M. and Strey, R. (1995) Phase behavior of the quaternary system H20/ -octane/C8E5/ -octanol Role ofthe alcohol in microemulsions./. Phys. Chem.,99,10313-10318. 

Nilsson, M., Sodermann, O. and Johansson, I. (2006) The effect of polymers on the phase behavior of balanced microemulsions Diblock-copolymers and comb-polymers. Colloid Polym. Sd., 284, 1229-1241. 

For these transient networks formed by the interaction of an ABA triblock copolymer and a microemulsion it has been shown that their principal viscoelastic properties are not affected significantly by the chemical nature of the microemulsion, i.e., they are similar for systems with both nonionic and ionic surfactants. Also it should be noted that the phase behavior of the corresponding microemulsion is qualitatively preserved, i.e., the reversible aggregation of the nanodroplets and the phase transitions to lyotropic liquid crystalline phases remain essentially unchanged (although the concentrations at which they occur might... 

Beckman et al. observed an effect of the secondary microemulsion structure on the molecular weight and yield of the polymer. Under conditions where extensive micelle-micelle clustering occurred, at lower fluid density the molecular weight of the polymer was as much as two times higher. Thus, the density of the supercritical phase could be used to control the polymer morphology. Beckman and Smith also completed an extensive study [74] of the effect that acrylamide, surfactant, and water concentrations as well as the pressure and temperature had on the phase stability of the microemulsions. The phase behavior of these systems depends on the choice of operating parameters, and this behavior can be exploited to optimize the properties of the polymer. 

Table 1 Variables for Tuning the Phase Behavior of Ionic Microemulsions...

Phase behavior studies are typically conducted in small (up to 100 mL) vials in order to determine what type, if any, of microemulsion is formed with a given micellar-cmde oil system. The salinity of the micellar solution is usually varied around the salt concentration of the field brine where the process will be applied. Besides the microemulsion type, other factors examined could be oil uptake into the microemulsion, ease with which the oil and aqueous phases mix, viscosity of the microemulsion, and phase stability of the microemulsion. 

In order to understand this complex relaxation behavior of the microemulsions, it is necessary to analyze dielectric information obtained from the various sources of the polarization. For a system containing more than two different phases the interfacial polarization mechanism has to be taken into account. Since the microemulsion is ionic, the dielectric relaxation contributions are related to the movement of surfactant counterions relative to the negatively charged droplet interface. A reorientation of AOT molecules, and of free and bound water molecules, should also be mentioned in the list of polarization mechanisms. In order to ascertain which mechanism can provide the experimental increase in dielectric permittivity, let us discuss the different contributions. 

In a recent report, the phase equilibria of a micromnulsion were reported (Birdi, 2010a). Phase behavior of a microemulsion formed with food grade surfactant sodium bis-(2-ethylhexyl) sulfosuc-cinate (AOT) was studied. Critical microemulsion concentration was deduced from the dependence of pressure on CP on the concentration of surfactant AOT at constant temperatme and water concentration. The results show that there are transition points on the CP curve in a very narrow range of concentration of surfactant AOT. The transition points were changed with the temperature and water concentration. These phenomena show that lower temperatme is suitable to forming microemulsion droplet and the microemulsion with high water concentration is likely to absorb more surfactants to structure the interface. 

Microemulsions are defined as dispersion of either water in oil or oil in water aided by mixed amphiphiles to produce transparent, isotropic, low viscous, thermodynamically stable solution systems. A microheterogeneous system of microemulsion with nanodispersion of one liquid into another must have a very low (about zero) interfacial tension caused by the presence of amphiphiles at the interface. In addition, the interfacial region must be highly flexible, either to permit the curvature required to surround exceedingly small particles or to allow the easy transition from oil-continuous to water-continuous structure, which is an important physicochemical feature of microemulsion. Along with the concept of nanodispersed droplets of either water in oil or oil in water, there is another concept of simultaneous dispersion of both water and oil in a system termed as bicontinuous where both the liquids are dispersed nearly in equal proportions. A microemulsion system can be one of the three types depending on the relative ratios of the constituting components. To achieve formation of any such system, phase behavior of the multicomponent (oil, water, amphiphile) systems is required to be studied, which is complex and has been elaborately... 

From the series of ILs tabulated in Table 15.1, we have chosen [C mim][BFJ to further characterize the microemulsion. We have characterized the partial phase behavior of the ternary system [C mim][BF4]/[C4mim][AOT]/benzene by observing the transition from clear transparent solution to tnrbid solution in naked eye (Fig. 15.1a). Based on the phase diagram, a series of samples were chosen where we can... 

Han and coworkers [38] determined the phase behavior of the ternary system consisting of [bmim][PFJ,TX-100, and water at 25 °C. By cyclic voltammetry method using potassium ferrocyanide, K Fe(CN)g, as the electroactive probe, the water-in-[bmim][PFJ, bicontinuous, and [bmim][PFJ-in-water microregions of the microemulsions were identified (Fig. 16.7). The hydrodynamic diameter of the [bmim] [PFJ-in-water microemulsions is nearly independent of the water content bnt increases with increasing [bmim] [PF ] content due to the swelling of the micelles by the IL. Sarkar and coworkers [39-41] reported the solvent and rotational relaxation studies in [bmim][PFJ-in-water microemulsions and water-in-[bmim][PFJ microemulsions using different types of probes, coumarin 153 (C-153), coumarin 151 (C-151), and coumarin 490 (C-490). The solvent relaxation time is retarded in the IL-in-water microemulsion compared to that of a neat solvent. The retardation of solvation time of water in the core of the water-in-IL microemulsion is several thousand times compared to pnre water. Nozaki and coworkers [42] reported a broadband dielectric spectroscopy study on a microemnlsion composed of water. 

---